Sen. Ben Sasse (R-Neb.) will not vote for Donald Trump if he is the Republican presidential nominee, the Nebraska Republican declared in a Facebook post on Sunday night.

He is the first Republican senator to make such a declaration.

Sasse, who has not shied away from speaking out against his party’s frontrunner, wrote that he would most likely seek out “some third candidate – a conservative option, a Constitutionalist” if Trump earns enough delegates to clinch the Republican nomination.

Citing, in particular, Trump’s inability earlier Sunday to disavow David Duke and the Ku Klux Klan just two days after he explicitly disavowed him, Sasse blasted Trump’s “relentless focus … on dividing Americans, and on tearing down rather than building back up, this great nation.”

“My current answer for who I would support in a hypothetical matchup between Mr. Trump and Mrs. Clinton is: Neither of them,” he continued. “I sincerely hope we select one of the other GOP candidates, but if Donald Trump ends up as the GOP nominee, conservatives will need to find a third option.”

“I do not claim to speak for a movement, but I suspect I am far from alone. After listening to Nebraskans in recent weeks, and talking to a great many people who take oaths seriously, I think many are in the same place,” the senator wrote. “I believe a sizable share of Christians – who regard threats against religious liberty as arguably the greatest crisis of our time – are unwilling to support any candidate who does not make a full-throated defense of the First Amendment a first commitment of their candidacy.”

On Friday, Trump declared that he would “open up” libel laws in the United States to make it easier to sue media organizations, and he has repeated that threat in subsequent speeches.

“Conservatives understand that all men are created equal and made in the image of God, but also that government must be limited so that fallen men do not wield too much power,” Sasse wrote. “A presidential candidate who boasts about what he’ll do during his ‘reign’ and refuses to condemn the KKK cannot lead a conservative movement in America.”

Sasse concluded by his own opinion on how to “make America great again.”

“We need more people engaged in the civic life of our country—not fewer. I genuinely appreciate how much many of you care about this country, and that you are demanding something different from Washington. I’m going to keep doing the same thing,” he wrote. “But I can’t support Donald Trump.”